The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the UMTS (Universal Mobile Telecommunication Service) system, and LTE is currently under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). Thus work is ongoing in 3GPP to specify an evolution to UTRAN, denoted E-UTRA, as part of the LTE effort. The first release of LTE, referred to as release-8 (Rel-8) can provide peak rates of 300 Mbps, a radio-network delay of e.g. 5 ms or less, a significant increase in spectrum efficiency and a network architecture designed to simplify network operation, reduce cost, etc. In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least frequency division duplex (FDD) and time division duplex (TDD). Other operation modes can also be used. The modulation technique used in LTE is known as OFDM (Orthogonal Frequency Division Multiplexing).
For the next generation mobile communications system e.g. IMT-advanced and/or LTE-advanced, which is an evolution of LTE, support for bandwidths of up to 100 MHz is being discussed. One issue with such wide bandwidth is that it is challenging to find free 100 MHz of contiguous spectrum, due to that radio spectrum a limited resource.
LTE-advanced can be viewed as a future release, denoted release-10 (Rel-10) of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-advanced can be deployed in spectrum already occupied by LTE (e.g. Rel-8). This means that for an LTE user equipment or a LTE terminal, a LTE-advanced capable network can appear as a LTE network. In both LTE and LTE-advanced radio base stations known as eNBs or eNodeBs—where e stands for evolved—, multiple antennas with beamforming technology can be adopted in order to provide high data rates to user equipments.
As mentioned earlier, LTE-advanced can support 100 MHz of bandwidth. This can be performed by aggregating non-contiguous spectrum, to create, from e.g. a baseband point of view, a larger system bandwidth. This is also known as carrier-aggregation, where multiple component carriers are aggregated to provide a larger bandwidth. The scalable bandwidth makes it more difficult to ensure that the overall channel response of the radio frequency (RF) chain of a eNodeB does not suffer from significant variations over frequency of the communication channel. If the channel response of the RF chain is not properly dealt with, the system may suffer from a substantial increase of frequency-selectivity as well as the performance of beamforming or pre-coding. Another concern relating to frequency selectivity and it impact on the performance of a system is the so called group delay. In general, the group delay is defined as a measure of the transit time of a signal through a device/apparatus/component versus frequency. The impact of group delay is more pronounced for wideband systems than in small band systems due to the substantial increase of frequency-selectivity. This will impact on antenna calibration.
In prior art technical documentation G. Tsoulos, J. McGeehan and M. Beach, “Space division multiple access (SDMA) field trails. Part 2: Calibration and linearity issue,” IEE Proc-Radar, Sonar Navig., Vol. 145, No. 1, February 1998; It is shown that there is approximately 2-3 dB reduction in the null depth between the 0 and 1% carrier frequency bandwidth case, however with 10% bandwidth the null is approximately 23 dB and 26 dB compared to that with the 0% bandwidth case. For example, if the carrier frequency is 2 GHz, the allowable frequency-independent bandwidth is less than 20 MHz. For systems with a bandwidth with more than 40 MHz, calibration is required for digital beamforming system in different frequencies. In order to achieve reliable and good system performance, calibration or antenna calibration is thus required. Especially for wideband systems such as LTE or TDD-LTE, the calibration across the transceiver RF chain of a radio base station e.g. a eNodeB is important for achieving adequate channel reciprocity and for effectively exploiting the channel reciprocity, since RF calibration mismatch degrades the reciprocity and impacts on the antenna gain(s). It should be mentioned that channel reciprocity means that the upstream (or uplink) and downstream (or downlink) channels are essentially the same.
As mentioned earlier, group delay impacts on the performance of a system. In order to reduce or eliminate negative this effect, group delay should be detected and dealt with properly. An interface known as CPRI (Common Public Radio Interface) comprised in a radio base station is generally used to detect a delay, but only the delay which is induced by e.g. cable length which can be detected and calibrated for by means of the CPRI as described in “CPRI specification Interface Specification, February 2009”. In other words, the CPRI interface is not suitable to use for detecting the group delay and for calibrating for group delay induces in the radio base station. This also means that when the available wide bandwidth is divided into multiple frequency groups (or subbands) such as in the OFDM based-system LTE or LTE-advanced, the induced group delay of each subband cannot be detected and calibrated using the CPRI interface.